Ni<sup>2+</sup>-Rich\nCollagen/Lignin Composite Hydrogel:\nTransforming Industrial Waste Materials into Flexible Electronics

Polymer-based conducting hydrogels have drawn significant\ninterest\nfor supercapacitors because of their fascinating features, including\nexcellent conductivity, tunable mechanical properties, porous structure,\noutstanding flexibility, scalable processability, environmental friendliness,\nand low production cost. Herein, a dynamic redox process was designed\nutilizing collagen (CG), poly(acrylic acid) (PAA), lignosulfonate\n(LS), and Ni<sup>2+</sup> to synthesize CG/PAA/LS/Ni hydrogel. The\nhydrogel’s unique features, including high ionic conductivity\n(IC) (4.89 S/m), outstanding flexibility, and stretchability, were\nassigned to the effective complex formation of Ni<sup>2+</sup> with\nthe numerous functional groups of CG, LS, and PAA. With a maximum\ntensile strength of approximately 0.61 MPa at an elongation of 1595%\nand a maximum compressive strength of ∼208 kPa with the highest\nstretchability of 65%, the CG/PAA/LS/Ni hydrogel demonstrated exceptional\nmechanical properties. The prepared hydrogel can also monitor human\nmotion with high sensitivity. The supercapacitor, assembled from the\nCG/PAA/LS/Ni hydrogel, manifested specific capacitance (C<sub>s</sub>), highest energy density (E<sub>d</sub>), and power density (P<sub>d</sub>) of 245.6 F/g, 27.63 Wh/kg, and 2.7 kW/kg, respectively.\nEven after 5000 consecutive cycles of charging and discharging, the\nsupercapacitor can retain its capacitance of 95.4%. This study opens\nup possibilities for the effective use of industrial waste in flexible\nelectronics.